As soon as the thread enters the passage area transversely to its longitudinal direction, it will reflect part of the incident light in essentially undefined directions due to its three-dimensional shape and its irregular surface. Part of the light reflected will fall on the receiver, which will then produce a signal representative of the presence of the thread. On the other hand, the sensor can also be constructed such that the receiver will produce a signal representative of the absence of the thread, when no thread is present. Especially in a thread storage and feed device in which a thread supply consisting of non-separated thread windings is produced and advanced approximately transversely to the longitudinal direction of the thread, the sensor will, for example, scan the size of the thread supply so as to be able to control with the aid of the signals produced during such scanning either the supplementation of the thread supply or an interruption of the supplementation of the thread supply (maximum sensor). It is, however, just as well possible to detect by means of the sensor the moment at which the thread supply has reached a predetermined reference position on the storage surface and to derive a signal therefrom (reference sensor). The reliability of the sensor depends on the possibility of producing strong and unequivocal signals on the basis of the presence or absence of the thread, and, in this respect, the reliability has hitherto been impaired by interfering optical influences in or at the passage area. Such interfering influences may by lints, which can not practically be avoided when threads are being processed, or some type of surfaces or objects which, from the incident light reflect light which falls onto the receiver.
In the case of an optical sensor, which is provided in a thread storage and feed device and which is known from EP-A1-03 27 973, the element whose surface is in contact with the thread is a reflector consisting of a reflective strip embedded between light-transmitting glass layers. The reflector is positioned in an axially extending cavity of a finger of the finger or rod cage. The light source is combined with a receiver responding to that part of the light of the light source which is reflected by said reflector and using reflected light variations, which are caused by the thread passing through, for the purpose of signal generation. Scattered light reflected by the thread or by lints or by extraneous elements is, in comparison with the light reflected by the reflector, too weak for generating a signal. However, lints or other extraneous elements in the passage area may cause the same reflected light variation as the thread, and, consequently, they may result in an incorrect signal.
In the case of an opto-electronic sensor provided in a thread storage and feed device and known from WO90/06 504, a sharply defined image of the thread in the passage area is produced by means of depth of field limitation with the aid of an image-forming optics and is then used for signal generation, said forming of an image and said signal generation being carried out irrespectively of whether there is any surface on the passage area side facing away from the light source and, if this is the case, irrespectively of the kind of surface. On the contrary, the scattered light reflected by the thread from the light of the light source is used for producing the sharp image.
In the case of an opto-electronic sensor of a colour selector of a multicolour weaving machine (FR-A-24 47 416), the colour selector enveloping surface, which is arranged behind the thread, is black or provided with a black covering so that no reflected light will be guided to the receiver. The thread moves in the longitudinal direction and above the enveloping surface in spaced relationship therewith.
The Patent Abstracts of Japan, Vol. 6, No. 158 (C-120) (1036), Aug. 19, 1982, discloses that, upon spinning a thread from fibres, the difference in the scattered light reflection between a thread running regularly through a suction tube in the longitudinal direction and fibre tufts passing through said tube after breaking of the thread is scanned for the purpose of detecting malfunction. The interior wall of the suction tube is black so that any light originating from a light source, which is directed onto the suction tube through a slot in the tube wall, will be absorbed.
U.S. Pat. No. 3,430,426 discloses that, in the case of an optical sensor in a spinning frame, a black body is provided as a background wall, said black body reflecting little, or no light at all to a group of receivers for supervising thus the correct operation of the spindles.
The present invention is based on the task of providing a sensor of the type mentioned at the beginning by means of which incorrect signals caused by light reflected by objects other than the thread are avoided.
According to the invention, due to the light-absorbing structural design of the surface of the element, only reflected scattered light from the thread will fall onto the receiver, which will derive strong and unequivocal signals from the presence or from the absence of the thread in the passage area. The fact that the element is constructed as a shield element prevents the generation or transmission of light reflected by surfaces or objects other than the thread, which are located on the other side of the passage area, and falling onto the receiver, so that no incorrect signals will be generated. Especially in thread storage and feed devices, lints or lint tufts will accumulate in unavoidable cavities of the rod or finger cage; said lints or lint tufts have a reflection behaviour which is similar to that of the thread itself, and they may emit, from the light coming from the light source, reflected scattered light resulting in incorrect signals. The shield element shields off dirt, which may perhaps be present in the passage area and behind the thread (lints, lint tufts or extraneous dirt), or reflecting surfaces, so that light from the light source will not fall onto said dirt or reflecting surfaces. Also light which has been generated in some other way and which is, in principle, directed onto the receiver is prevented from falling onto said receiver. It is true that the shield element does not generate any direct and strong reflected light at its light-absorbing surface; since, however, even light-absorbing surfaces generate, in practice, still some amount of reflected light, the receiver will be arranged outside of the area of the scattered light reflected by the surface, and light reflected by objects other than the thread will thus be prevented from being applied to the receiver in an undesirable manner. Dirt is prevented from adhering to the surface, since said surface is permanently cleaned by the thread.
In one embodiment, the reflected light, which cannot be avoided at the light-absorbing surface, will reliably bypass the receiver so that said receiver can concentrate on the scattered light reflected by the thread and so that it will produce its signals only in the presence or in the absence of the thread. A very distinct signal transition (signal shape) between the presence and the absence of the thread is achieved, and this will facilitate the evaluation of the signals.
In the invention, a plastic plate can easily be accommodated in the thread storage and feed device below the passage area.
Also in the invention, the lints, lint tufts or extraneous dirt, which will inevitably accumulate in the cavity, will have no chance of generating reflected light from the light of the light source. Light which does not originate from the thread and which is directed towards the receiver from below in some other way will be shielded off as well.
Further in the invention, the surface will only generate reflected light having an oblique exit angle--if it generates any reflected light at all due to the oblique incident angle of the light. Due to the fact that the receiver is arranged perpendicularly to the surface of this antimirror (blind mirror) and above the area in which the light falls onto the surface, this reflected light will bypass the receiver. At least part of the unoriented reflected light of the thread will, however, fall onto the receiver in the predetermined way. The scanning direction perpendicularly to the longitudinal direction of the thread results in a distinct and easily scannable signal transition (signal shape) at the receiver. However, in specific cases of use, a scanning direction parallel to the longitudinal direction of the thread or even obliquely to said longitudinal direction may be expedient. In practical operation, the respective scanning direction chosen will be determined by the space conditions and by the possibilities of applying a strong reflected light component from the thread to the receiver.
In another aspect of the invention, the thread causes permanent automatic cleaning of the light-absorbing surface so that the light-absorbing properties will be preserved for a long time. Furthermore, the thread is precisely guided on the surface during the scanning operation.
Alternatively, the fibres sweeping over the surface suffice to produce the cleaning effect, whereas the thread itself is not subjected to any perceptible frictional forces.
In another aspect of the invention, the shield element permits light which comes from the light source to pass, whereas, in the opposite direction, it does not permit such passage of light. This has the effect that a particularly effective light absorption is achieved. The shield element is constructed similar to the glasses of a mirrorred pair of sunglasses.